Flexographic plates serve as printing plates in a variety of applications. As an example, flexographic plates can be used for printing patterns on plastic bags, such as bags containing food products. Further, flexographic printing can be applied on relief material, such as labels, card board for packaging products and corrugated (card)board or paper.
Prior to an actual printing phase, a fresh flexographic plate is processed for making a desired relief pattern, including a step of providing a floor layer on top of the carrier layer and including a step of imaging photosensitive material in the relief layer of the plate, using a mask technology, such that said material locally polymerizes. Then, un-polymerized material is removed in a washing/cleaning process, so that the polymerized relief pattern is left behind and protrudes upwardly from the carrier layer and from the floor layer. The process may include either a water based washing process, a solvent based washing process, or a heating process. During the printing process, the protruding pattern contacts a surface to be printed. The height of the relief pattern structure on the plate serves to selectively pick up ink and transfer it to the material to be printed. The substantial advantages of flexo printing are the flexibility in printing materials and the simplicity and stability of the printing process on less sophisticated printing machines than necessary for the offset printing process.
The optical system of a known processing apparatus includes an IR (infra red) light source, for generating IR light, typically at a wavelength of circa 830, nm, circa 940 nm or circa 1070 nm, for locally ablating the carbon layer to form a mask. The IR light source is e.g. a high power IR laser to enable carbon particles of the carbon layer to evaporate. The optical system of a known processing apparatus also includes a UV light source, such as a trans-luminance lamp, for causing the photosensitive material in the relief layer to start a polymerization process upon incidence of a UV light beam, typically UVA, UVB and UVC. The UV light propagates into the relief layer after transmission through mask apertures.
It has been observed that, in the process of forming the carbon mask, impurities may be encapsulated in the photosensitive material causing optical loss of quality during imaging. Among others, oxygen particles may diffuse into the relief layer causing a termination of a polymerization process that has been initiated by the UV light beam. This process is known as oxygen inhibition. Since additional oxygen particles may enter the relief layer, via the mask, the final polymerized structure will not reach its full height. Especially near the top of the polymerized structure and near boundaries of the mask apertures, incomplete polymerization may occur, thus developing polymerized structures having a moderately defined top profile. As an example, a square dot image is printed by a pillar, in the relief pattern, having a top profile approaching a semi sphere. Generally, the top profile is not flat but includes a rounded, curved surface causing inferior printing features.
Relief structures including curved, rounded top segments have a number of disadvantages. The area that is printed with such relief patterns depends on the pressure that is exerted when bringing the flexographic plate in contact with the substrate to be printed. Generally, when the pressure force increases, also the printed area increases, thus allowing visible variations on the printed substrate when printing parameters are within a reasonable range of operation. During life time of the flexographic plate, wear of the relief pattern modifies the printing area considerably since the contact area between the flexographic plate and the surface to be printed increases over time.
In prior art systems, different approaches are known to cope with the above-mentioned effects. As an example, lamination techniques are applied. Also, oxygen is removed from the atmosphere during curing conditions. Further, a high intensity UV surface crosslinking process is induced by applying high power UV radiation sources. However, all these approaches inherently have other drawbacks such as sensitivity to dust, complex, expensive devices etc.